1. Field of the Invention
The present invention relates to processes for upgrading heavy viscous hydrocarbons, such as viscous crude oils, bitumens from tar sands, hydrocarbons derived from coal, lignite, peat or oil shale, residuum resulting from the atmospheric and/or vacuum distillation of lighter crude oils, heavy residues from solvent extraction processes, and the like. Such processes include, for example, the treating to reduce the viscosity of heavy viscous crudes which are impractical to pump at ambient temperatures to obtain a product which is practical to pump through conventional pipe lines. Additionally, some of the upgrading processes include reducing the metals, particularly nickel and vanadium, and Conradson carbon content while reducing the specific gravity.
2. Description of the Prior Art
A large number of processes are available for treating heavy, viscous hydrocarbons, such as Boscan crude from Venezuela or Cold Lake crude from Canada, to obtain an upgraded product with lower viscosity, specific gravity, metals content, and Conradson carbon content. Generally these processes may be grouped into two broad classes: (1) the solvent extraction processes which remove high carbon viscous materials and (2) the conversion processes.
The solvent extraction processes rely on physical separation, not chemical conversion. In a typical three-stage solvent extraction process where oils, resins, and asphaltenes are produced as separate fractions, the metals, sulfur, and Conradson carbon contents are highest in the asphaltene fraction, next highest in the resin fraction, and smallest in the oil fraction. The relative amounts of the asphaltene, resin and oil fractions and the corresponding properties thereof, can be varied over a wide range by changing solvents and operating conditions in the solvent extraction unit. When producing a minimum amount of the asphaltene fraction, it generally happens that the metal and Conradson carbon content of the resin fraction is usually increased to the point where the resin fraction is not a desirable material for subsequent catalytic processing such as catalytic cracking or hydrocracking.
In order to produce a solvent extracted oil with acceptable metal and Conradson carbon levels for catalytic processing, it is necessary to limit the yield of the oil fraction and increase the yields of the resin and asphaltene fractions. Since the latter two fractions generally must be used as a residual fuel of very low value, a serious economic penalty on the utilization of solvent extraction processes results.
Similar results are obtained with a two-stage solvent extraction unit. The two-stage unit may be operated to include the resins in varying degrees with the asphaltenes or with the oils. The metals and Conradson carbon contents of the fractions would vary accordingly. It is also possible to operate four or more stages of a solvent extraction unit. Varying cuts can be made depending on operation with the heaviest cuts containing the highest molecular weight materials, the greatest viscosity, and the highest metals and Conradson carbon content.
The second broad class includes processes which convert the high boiling viscous hydrocarbons to lighter products. These conversion processes can be grouped into three categories: (1) processes which employ a high hydrogen partial pressure; (2) thermal cracking processes which prevent coke formation by special design and by limiting conversion; and (3) processes which produce coke.
The thermal cracking processes are generally less expensive than those in the other categories but generally produce a lower yield of residual and gas-free products. "Residual and gas-free products" are defined herein as total products, less (1) C.sub.2 and lighter gas, (2) coke, (3) liquid boiling above 1050.degree. F. containing more than 10% Conradson carbon, and (4) Conradson carbon content of other products. The yields of thermal cracking processes are limited by feedstock quality, product quality, and coke formation. For a given feedstock, the greatest conversion may be obtained by increasing the severity to the level where the product quality or rate of coke formation become unacceptable. The rate of coke formation is increased as the resins and high moleculer weight oils, which act to peptize and maintain the asphaltenes dispersed, are cracked. This causes the asphaltenes to become incompatible with the surrounding constituents, to start to form a sediment, to increase in number and/or size due to polymerization and/or condensation reactions, and to increase the rate of coke formation. This also affects the quality of products from thermal cracking processes as the asphaltenes and sediments detract from product quality by adversly affecting product stability and compatability with blending stocks.
Hydroconversion processes generally produce the highest yield of residual and gas-free products, but are also much more costly both from an investment and an operating cost standpoint than thermal cracking processes. The hydroconversion processes require a high investment because a hydrogen production facility is required to supply hydrogen and high hydrogen partial pressure is required in the hydroconversion unit to either suppress coke formation on the catalyst or to accomplish the hydrogen addition noncatalytically. Utilities costs for typical hydroconversion processes are high because of the high cost of hydrogen compression and the multiplicity of steps involved. Additionally, operating costs are increased where high metals content of heavy crudes such as Boscan and Cold Lake result in catalyst deactivation.
In a typical hydroconversion process, the crude is usually subjected to successive atmospheric and vacuum distillation to reduce the amount charged to the very expensive high pressure residual hydroconversion step. This hydroconversion requires that the bottoms from the vacuum distillation be further heated to hydroprocessing reactor temperature. Part of the effluent from the hydroconversion reactor is then cooled to produce a hydrogen recycle stream with low hydrocarbon content. The remaining effluent is then further heated for distillation and followed, in some cases, by solvent extraction to produce a heavy residual together with gas oil and lighter products. These repeated heating and cooling steps result in relatively high investment and operating costs.
Processes such as delayed and fluid coking can be heat integrated to avoid repeated successive heating and cooling steps. However the yield of residual and gas-free products of such coking processes are generally less than hydroconversion processes. Further the olefinic content as indicated by the bromine number of coking products is usually relatively high resulting in a high hydrogen consumption in subsequent refining processes to produce finished products.